tests/avocado: exec_command should not consume console output

tests/avocado: exec_command should not consume console output

_console_interaction reads data from the console even when there is only
an input string to send, and no output data to wait on. This can cause
lines to be missed by wait_for_console_pattern calls that follows an
exec_command. Fix this by not reading the console if there is no pattern
to wait for.

This solves occasional hangs in ppc_hv_tests.py, usually when run on KVM
hosts that are fast enough to output important lines quickly enough to be
consumed by exec_command, so they get missed by subsequent wait for
pattern calls.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240805232814.267843-2-npiggin@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>